Optical encoders or tachometers are well known in the art. Basically, these devices comprise a stationary light source arranged to be mounted on a motor close to the motor shaft, a stationary receiving photoelectric cell in alignment with the light source and a rotating shutter arranged to be secured to the motor shaft to rotate between the light source and photo cell and thus optically generate a series of pulses of frequency corresponding to the RPM of the motor. In these assemblies, it is vitally important that the light source and photo cell be properly aligned with respect to the rotating shutter in order that usable output pulses are properly generated. Any misalignment could result in the absence of pulses or very weak pulses if the full apertures defined by the rotating shutter are not utilized.
There is generally no problem when an optical encoder is installed on a motor in the factory. Proper alignment jigs and the like are readily available and once the installation is complete on the motor, there is ordinarily no problem of misalignment unless an ultimate user attempts to disassemble the optical encoder from the motor and attempts to reassemble the same himself. On the other hand, it is presently common practice to provide optical encoder kits which may be shipped directly to a user and assembled by the user himself on a motor. Normally, the basic component of the assembly constitutes a mounting member carrying the light source. This member is provided with a central bore normally of diameter greater than the motor shaft diameter of any particular motor with which the encoder is to be used. The larger diameter central bore is necessary to provide a definite clearance between the rotating shaft and the mounting member carrying the light source which latter member is fixed to the motor and stationary. The next component of the assembly constitutes the shutter member which is mounted directly on the motor shaft. Finally, an appropriate photocell detector overlies the shutter member and is affixed to the mounting member.
Because of the larger diameter central bore of the mounting member relative to the motor shaft, in affixing the mounting member to the motor in a stationary position, it is essential that it be exactly coaxially centered relative to the motor shaft. Otherwise, the rotating shutter will be out of alignment with the light path from the light source to the photo cell all as described heretofore.
In view of the foregoing, it is customary to provide a centering means such as a bar with a V-groove affixed to the mounting member at the factory to permit exact centering of the mounting member prior to securing it to the motor. After securement of the mounting member, this centering bar or fixture can be disconnected. However, if it is ever desired to remove the optical encoder and place it on another motor or even reassemble it on the same motor, the mounting member must be sent back to the factory for proper securement of the bar fixture thereto in order that proper centering can again be achieved.